API vs. Web Services: What’s the Difference?

Short answer: An API (application programming interface) is a defined way for software components to communicate. A web service is a network-accessible service interface, usually reached with web technologies. In broad modern usage, web services are a subset of APIs: every web service is an API, but an API does not have to be a web service.

The labels answer different questions. “API” describes the interface and its contract. “Web service” describes an interface available across a network. REST and SOAP add another dimension: REST is an architectural style, while SOAP is a messaging protocol. They are not synonyms for API and web service.

1. API and web service definitions

What is an API?

An API is the set of definitions, rules and protocols that lets one software component request behavior or data from another. The components may run in the same process, on the same computer or on separate machines. AWS describes APIs as mechanisms that let software components communicate through definitions and protocols (AWS API explanation).

A standard-library function is an API. So is a database driver, an operating-system system call, a JavaScript module, or an HTTPS endpoint. The API contract may specify function names, parameters, return values, errors, authentication and versioning.

What is a web service?

A web service is a service interface exposed over a network for other software to consume. HTTP or HTTPS is common, but the term can also cover services that use other messaging transports. In standards-era writing, “web service” often referred to a formally described service exchanging SOAP messages and using web standards. The W3C Web Services Architecture document describes services that interact according to a service description and can map to different languages, platforms and messaging systems.

Contemporary engineering teams frequently use “web service” for any remotely callable HTTP service. That broad usage is useful, but terminology varies by organization and by era.

Question	API	Web service
What does the term emphasize?	The software interface and contract	Network availability of the service
Must it use a network?	No	Yes, by definition
Must it use HTTP?	No	Usually, but not always
Typical examples	Library function, SDK, system call, HTTP endpoint	REST endpoint, SOAP service, JSON-RPC endpoint

2. The relationship: subset, not a one-to-one match

Use a set diagram mentally: APIs are the larger set. Web services occupy the network-accessible portion of that set. A local image-processing library exposes an API without being a web service. A remote endpoint that returns an invoice is both an API and, commonly, a web service.

This relationship is a practical shorthand rather than a timeless formal definition. AWS calls web services a special type of API, while the W3C document uses a narrower standards framework. When documenting a system, define what your team means instead of assuming every reader uses the same vocabulary.

Are all web services APIs?

Yes, under the normal software-engineering meaning: a web service must expose an interface that clients can call, so it is an API. If someone uses “web service” to mean an entire deployed application, ask which callable interface they mean.

Are all APIs web services?

No. A function imported from a local package, a browser DOM API and a kernel system call are APIs that do not expose a network service.

Is a REST API a web service?

Usually. REST describes constraints on how a network interface is designed; “web service” describes the fact that the interface is remotely available. A RESTful HTTP endpoint can accurately be called both a REST API and a web service.

3. Keep API, web service, REST and SOAP separate

Confusion starts when four different concepts are treated as competing labels:

• API: an interface contract between software components.
• Web service: a network-accessible service interface.
• REST: an architectural style built around constraints such as resources, representations and stateless interactions.
• SOAP: a messaging protocol with structured envelopes and commonly XML-based messages.

A REST API may be called a RESTful web service. A SOAP service exposes an API through SOAP messages. Neither statement means API equals REST or web service equals SOAP. AWS’s comparison explains SOAP as a protocol and REST as an architectural style (AWS SOAP and REST); the European Commission’s JRC study makes the same distinction (JRC Web APIs and Web Services).

Combination	Valid?	Why
Local library API	Yes	An API does not require a network.
REST web service	Yes	REST can describe a network service.
SOAP web service	Yes	SOAP messages can expose a network service.
REST API that is not a web service	Sometimes	REST constraints can be applied outside a conventional web deployment, although HTTP REST APIs are the common case.

4. SOAP does not have to mean HTTP

HTTP is common for SOAP, but it is not a requirement of the protocol. CMS documentation lists HTTP, AMQP and proprietary messaging protocols as possible transports for SOAP messages (CMS SOA Concepts). Therefore, avoid the shortcut “SOAP is XML over HTTP.” A more accurate statement is “SOAP commonly uses XML messages and can be carried over several transports.”

REST is also not simply “JSON over HTTP.” JSON is a representation format; REST is a set of architectural constraints. Many REST APIs use JSON and HTTP verbs, but those implementation choices alone do not prove that an interface follows every REST constraint.

5. How to compare two real interfaces

When choosing an integration, compare concrete properties instead of labels:

1. Interface scope: Is it a local call, an internal network endpoint or a public service?
2. Transport: Does it use HTTPS, AMQP, a message queue, a Unix socket or another transport?
3. Message format: JSON, XML, form data, binary frames or another representation?
4. Contract: Is there OpenAPI, WSDL, a schema, generated client code or only prose?
5. Operations: How are authentication, retries, pagination, rate limits, idempotency and errors defined?
6. Compatibility: Does your existing client, gateway or governance system support the protocol?
7. Lifecycle: How are versions announced and deprecated?

The JRC taxonomy includes SOAP, XML-RPC, JSON-RPC and REST among web API approaches. That range is a reminder that “web API” is a family of interface styles, not one wire format.

6. A small HTTP API example

The following request illustrates the mechanics common to a web API: an endpoint, query parameters, an HTTP method and a response body. The endpoint is ScreenshotNeo’s website screenshot API. See the ScreenshotNeo documentation for the complete option list.

cURL

curl -G "https://api.screenshotneo.com/v1/shot" \
  -d access_key=YOUR_API_KEY \
  --data-urlencode url=https://stripe.com \
  -o shot.webp

Python

import requests

r = requests.get(
    "https://api.screenshotneo.com/v1/shot",
    params={"access_key": "YOUR_API_KEY", "url": "https://stripe.com"},
    timeout=90,
)
r.raise_for_status()
open("shot.webp", "wb").write(r.content)

Node.js

const q = new URLSearchParams({
  access_key: 'YOUR_API_KEY',
  url: 'https://stripe.com'
});
const res = await fetch(`https://api.screenshotneo.com/v1/shot?${q}`);
if (!res.ok) throw new Error(`HTTP ${res.status}`);
const bytes = new Uint8Array(await res.arrayBuffer());
await Bun.write('shot.webp', bytes);

This example is both an API call and a web-service call: the interface is defined by parameters and response behavior, and it is reached over HTTPS. The same conceptual API could also be wrapped by a Python or Node.js library, giving developers a second, local API.

7. Or skip the browser setup

If your goal is to obtain reliable screenshots rather than build a browser capture stack, ScreenshotNeo provides one GET request for PNG, JPEG, WebP or PDF output. Before capture it accepts cookie and consent banners and removes more than 60 known consent platforms, newsletter popups and chat widgets; each step can be disabled. Bot checks, blank pages, timeouts, failed loads and cache hits cost nothing, and the response identifies the result with X-Page-Verdict and X-Billed headers.

The API supports full-page capture with lazy images, CSS-element capture, dark mode, device presets, arbitrary viewports, retina scale, PDF paper and page options, custom CSS and JavaScript, clicks, selector waits, delays, network-idle waits, request and resource blocking, headers, cookies, user agents, authorization, timezone, geolocation, transparent backgrounds, resizing, configurable caching, signed image links, asynchronous jobs with signed webhooks, bulk capture of up to 100 URLs and a usage API. An MCP server exposes take_screenshot, get_page_info and capture_pdf to Claude, Cursor and other MCP clients.

There are 1,000 free screenshots each month with no card. Paid plans start at $5 for 3,000 screenshots; yearly billing gives two months free, and every feature is available on every plan. Create a free ScreenshotNeo account.

8. Troubleshooting terminology and integration errors

Symptom	Likely cause	Fix
“The API is not a web service.”	The terms are being treated as mutually exclusive.	Check whether the interface is network-accessible. If it is, it can be both.
“REST and API mean the same thing.”	An architectural style was confused with an interface category.	Describe the API first, then state whether it follows REST constraints.
“SOAP always uses HTTP.”	A common deployment was mistaken for a protocol requirement.	Check the transport contract; SOAP can use HTTP, AMQP or proprietary transports.
401 or 403 from an HTTP API	Missing, expired or incorrectly formatted credentials.	Check the authentication scheme, secret location and clock skew; never log secrets.
415 Unsupported Media Type	The request body format does not match the declared content type.	Set Content-Type correctly and send the format required by the contract.
404 from a service	Wrong base URL, version, path or HTTP method.	Compare the exact route and method with the provider’s specification.
Intermittent 5xx or timeouts	Transient provider, network or upstream failure.	Use bounded exponential backoff, respect retry guidance, and make retries idempotent.
Screenshot output is blank or blocked	The target requires a bot challenge, consent interaction or resources that failed to load.	Inspect verdict headers, wait conditions, cookies and blocked-resource settings; do not assume a successful HTTP status means useful content.

9. Performance, reliability and cost considerations

The words API and web service do not guarantee speed, uptime or security. Measure the actual operation and document limits. For HTTP integrations, reuse connections, set explicit timeouts, paginate large responses, cache safe reads and avoid retry storms. For asynchronous work, use a job identifier and a signed callback where the provider supports one.

Separate transport success from business success. An HTTP 200 can contain an application-level error or an unusable payload; validate status, content type, schema and required fields. Record request IDs and provider error codes without recording credentials or personal data.

Cost depends on the provider’s billing unit, cache behavior, retries and payload size. ScreenshotNeo bills only clean shots; cache hits and failed categories listed in its response are not billed. For any service, read the billing definition and model worst-case retries before production rollout.

10. FAQ

Does “API” always mean HTTP?

No. APIs can be local functions, SDKs, operating-system calls, message interfaces or HTTP endpoints.

Is a web service always public?

No. It may be private inside a company network, protected by a gateway or exposed only to selected clients.

Should I call my JSON endpoint a web service?

That is acceptable in broad usage. “HTTP API” or “REST API” is more precise when those details matter to the reader.

Which is better, SOAP or REST?

Neither is universally better. Compare the required contract, transport, tooling, security model, message format and existing system constraints.

Where do RPC APIs fit?

RPC treats the interface as remote procedure calls. JSON-RPC and XML-RPC are API approaches that can be delivered as web services when exposed over a network.

11. Key takeaways

• An API is the broad concept: a defined software interface.
• A web service is a network-accessible API, commonly reached through web technologies.
• REST is an architectural style; SOAP is a messaging protocol.
• SOAP is not limited to HTTP, and JSON over HTTP is not automatically REST.
• Choose integrations by contract, transport, format, operational behavior and cost—not by labels alone.